fx

Vercel · Terminal & CLI Agents · Updated Sep 15, 2026

fx is a tiny, native coding agent harness written in Zig, optimized for performance, research, and embeddability. It provides a Unix-like CLI experience and can be integrated into larger systems via its ACP server, WebAssembly builds, or one-shot command-line execution.

Recent Updates

Recent versions have optimized the shell and context compaction, introduced unified subagent delegation, improved MCP security with mandatory trust prompts, and added subscription-based authentication for Codex and Grok.

As of Sep 15, 2026

Key Features

Single-digit megabyte binary with microsecond cold starts
Model-agnostic support for local and cloud inference
Embeddable via WebAssembly, ACP, or native binary
Native support for MCP (Model Context Protocol) and reusable skills
Hierarchical subagent management with persistent and one-off sessions

Who It's For

✓ Best For

Researchers, developers building agent infrastructure, and teams needing a minimal, embeddable harness for CI pipelines, automated evals, or sandbox environments.

✗ Not Ideal For

Users seeking a feature-rich, out-of-the-box IDE replacement or a polished, stable product; it is explicitly labeled experimental and early-stage.

Pricing

Freemium

Open-source (Apache-2.0) with no charge for the harness; inference costs depend on the provider, utilizing Vercel AI Gateway, direct ChatGPT/Grok subscriptions, or local models.

Verdict

A high-performance, minimalist tool for those building agent-based systems, though currently experimental and best suited for infrastructure rather than as a primary daily-driver assistant.

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